Coral ReefsCoral ReefsShipley Marine BioShipley Marine Bio

Summit High SchoolSummit High School

Corals Corals are cnidarians,

a phylum of animals characterized by the presence of stinging cells on their tentacles

These stinging cells are used to capture prey primarily, but can also be defensive

The soft body of the coral (polyp) is enclosed in a hard shell of calcium carbonate created by the coral (whitish color in diagram on right).

Hermatypic corals-

Reef builders Normally contain zooxanthellae, symbiotic

photosynthetic dinoflagellates (shown at right) Restricted in distribution by water temperature

and water quality Ahermatypic corals-

Do not build reefs May not contain zooxanthellae Not restricted by water temperature and water

quality

Basic Types of Corals

Anatomy of a Coral Reef Coral reefs are

groups of coral polyps interconnected by thin layers of tissue

These grouped polyps share a nervous system and digestive connection – this is due to the fact that the entire colony came from one original polyp that reproduced asexually to produce the colony

Anatomy of a Coral Reef Each polyp

continues to lay down new CaCO3 beneath the polyp body so the entire coral colony continues to grow upward

Each polyp is normally 1 to 3 mm in width

However, the collection of polyps together can collectively grow to extremely large sizes and weigh several tons

The zooxanthellae housed inside the coral produce organic matter through photosynthesis

Some of this organic matter is passed to the coral This “sharing” aids the coral and helps the entire

reef grow faster In fact, if zooxanthellae are supplied with enough

light, they can feed the coral completely

Coral Nutrition

Of course, coral do also possess nematocysts to

sting potential prey that come within reach of the tentacles

These cells allow the coral to feed on small organisms in the water, mostly zooplankton

Some corals also produce sheets of mucous to capture zooxanthellae or detritus

Coral Nutrition

Coral Nutrition Coral can also feed

using extensions of the gut wall called mesenterial filaments

These filaments secrete digestive enzymes

The coral can extend the filaments with its enzymes allowing the coral to feed and digest food outside the body

Hard Substrate Light Optimal Narrow temperature range Narrow salinity range Low sediment load in water Low pollution Narrow range of pH

Conditions Required for Reef Growth

Corals reproduce both sexually and

asexually Asexual reproduction allows for the growth

of buds from a single polyp Sexual reproduction results in the creation

of a planula larvae that is able to ride on the water currents to a new environment

These larvae will settle out of the currents to begin growth on existing reefs or other hard substrate

Reproduction and Growth of New Coral

Light must be able to penetrate the water to the depth of a coral to allow the symbiotic zooxanthellae to photosynthesize

Due to this, corals are normally found in shallow waters

Some may be found to depths over 150 feet if water clarity permits light penetration to that depth

Corals are mostly found on continental shelves, near islands or on seamounts due to this light/depth dependency

Light Requirements

Reef building (hermatypic) corals tolerate a low

temperature range compared with most marine organisms

Corals can only reproduce and grow if water temperatures are over 68 degrees F

Higher water temperatures are preferred for optimal growth

However, water temperatures above 86 degrees (or higher in some species) causes coral stress

Coral stress can lead to “bleaching,” an expulsion of zooxanthellae from the coral polyp

Temperature Requirements

Coral Stress and Bleaching

The expulsion of zooxanthellae is termed bleaching because the normally colored coral is bleach white after this expulsion

The zooxanthellae give the corals their distinctive color

Other events besides high temperatures can cause coral bleaching

Poor water quality, increased sediment in water column, wave stress or disease can also cause bleaching

If corals remain bleached for too long, it can result in death of the coral

Widespread bleaching often occurs as a result of events such as El Nino (increased water temps) and hurricanes

Coral Stress and Bleaching

Normal sea salinity is around 35 parts per thousand

Corals maintain good health in an environment around 35 ppt

Corals normally do not exist near areas where rivers deliver large amounts of freshwater to the marine environment because of this salinity requirement

Low salinity can result in coral bleaching as well

Salinity Requirements

Corals due not normally grow in areas where wave action is high

This is due to the fact that heavy wave action results in the suspension of sediments in the water column

Suspended sediments can settle on corals causing damage as well as reduce water clarity

Reduced water clarity means that zooxanthellae photosynthesis will be reduced or stopped

Sediments and Wave Action

Even low levels of pollution can kill coral polyps High nutrient levels can also allow algae levels to

rise Increased algal growth can shade light-sensitive

corals and their zooxanthellae As an example, read the case study in chapter 14

in Kane’ Ohe Bay in Hawaii on eutrophication and the decline of coral reefs

Coral Sensitivity to Pollution

Coral colonies can come in a variety of shapes and sizes

Taller, more branching more tend to be more common in shallower areas of the reef due to intense competition for space and light

Flatter forms tend to be more common in deeper areas of the reef; this probably assists with capturing all possible light available.

Types of Coral Growth Forms

Types of Coral Growth Forms

Coralline algae (also produce calcium

carbonate) Soft corals such as sea whips and sea fans Other cnidarians such as hydrozoans or

anemones Sponges (help cement coral rumble into a

substrate) Bryozoans On the following page, there is a table that

outlines important cnidarians on coral reefs

Other Organisms that Contribute to Reef Structure

Besides Stony Corals

1.Fringing Reefs2.Barrier reefs3.Atolls

Types of Coral Reefs

Simplest and most common form of

reefs These reefs develop near the shore in

tropical waters These reefs develop as narrow strips

along the shore The fringing reef shown on the right

completely encircles the island it has formed near

Fringing Reefs

Fringing Reefs Fringing reefs

consist of an inner reef flat and an outer reef slope (shown at right)

The reef flat is wide, gently sloping and may be exposed in places at low tide

The reef slope is much more steep and is not exposed to air

The reef crest is the shallow, upper edge of the reef slope

Growth is normally high in this area

The reef slope contains living and dead

corals, seagrasses, soft corals, algaes and invertebrates

Growth of these organisms is normally fairly high on the reef slope and especially at the reef crest

The number of species present is also higher on the slope and the crest

Growth is slower on the reef flat due to runoff from land which includes sediment, freshwater and pollution

Wave action is normally higher in this area as well

Fringing Reef

Barrier reefs also grow along the shore

or may be farther from shore They are different from fringing reefs by

the presence of a deep lagoon between the shore and the reef

At right, you can see an aerial photo of the barrier reef in Belize, Central America – this is the second largest barrier reef in the world

Barrier Reefs

Barrier Reefs The illustration at right

shows the typical construction of a barrier reef: a back-reef slope, a reef flat and crest and a fore-reef slope

The slope of the fore-reef slope is normally much more pronounced than the back-reef slope

The depth of the water drops quickly in this area!

The slop of the back-reef slope may also be steep, but not to the degree of the fore-reef slope

Waves often wash sediment onto the

back reef slope or reef flat which causes reduced coral growth in this area

Enough sediment may accrue to form small islands on the reef

These islands are called keys in the US or cayes elsewhere

Barrier Reefs

Coral growth is normally the highest at

the reef crest or just below the crest on the fore-reef slope

As with fringing reefs, species diversity is greater in this area as well

Barrier Reefs

The Great Barrier Reef is considered the

largest reef structure in the world with regards to total coral area

The length of the reef systems exceeds 1200 miles

The width varies from 10 – 200 miles Many cayes exist along the reef system

Great Barrier Reef, Australia

Atolls An atoll is a circular

reef structure surrounding a central lagoon

Sand cayes may be a part of the atoll structure

Width varies from less than one mile to over 20 miles

Atoll Formation Atolls form when a

volcanic island is formed The coral begins as a

fringing reef around the newly formed island

Over time, weathering and geologic activity lowers the level of the island

Eventually, the island disappears below the water’s surface, leaving only the outer band of coral surrounding a lagoon

The diagram at right shows the progression of these steps

Atolls possess a reef flat and inner and outer

reef slopes The inner slope has a more gradual slope than

the outer slope and the lagoon normally has a depth of 200 feet or less

The two largest atolls are in the Maldives (Indian Ocean) and the Marshall Islands (Pacific)

Atolls

Like any marine environment, the coral

reef community is plagued by limiting resources, competition and predation

The water column surrounding the reef is normally very low in nutrients

This is due to the position of reefs away from rivers and other sources of nutrients (remember corals form best away from possible sedimentation sources)

Coral Reef Ecology

Coral Reef Ecology Due to this low nutrient

content, the water surrounding the reef does not support the degree of primary production by phytoplankton seen in other marine communities

The food chain here is based around the primary production of zooxanthellae located in the body of the corals and other organisms (in some cases)

The coral benefits the zooxanthellae by providing carbon dioxide, nitrogen and phosphorus (by-products)

The zooxanthellae provide organic molecules to the corals (and any organism that feeds on the corals or their parts

Cyanobacteria present on the reef are able to

fix nitrogen which is then available to other reef inhabitants who feed on the cyanobacteria

Seagrasses and algae can also feed reed inhabitants

Water currents may bring some phyto- and zooplankton to the reef community

Coral Reef Ecology

Typical Coral Reef Food Web

Many Organisms Feed Directly on Corals or their Products (mucous, eggs, larvae)

Many coral reef inhabitants can deter predation by producing toxic or foul tasting chemicals – this is common in soft corals and sponges

These chemicals can be released into the water to kill organisms or limit growth surrounding these organisms

Other reef dwellers may produce hard structures to deter predation – sponges produce spicules of calcium carbonate or silica while soft corals produce calcium carbonate needles known as sclerites

Coralline algaes as well as other organisms grow in an encrusting form that is difficult to feed upon (tissue layer is VERY thin)

Deterring Predation

Competition on the Reef

There is tough competition for space and light on the reef

To limit competition, some corals grow fast and upright to maximize their ability to reproduce quickly

Others grow slow but massive to out-compete other organisms in the long run

Corals may feed directly or sting one another if they contact each other

Shown at right is direct coral on coral competition (and some coral death in the pink area)

Remember that corals must also compete with algae, seagrasses and invertebrates for space and light

Competition on the Reef

Other organisms such as fish and crustaceans must also compete for limited resources

They normally do this by feeding in slightly different areas or in slightly different ways

Each has its own ecological niche